According to RFC 7296 (IKEv2) and Fortinet's official documentation, the IKE_SA_INIT exchange is responsible for negotiating cryptographic parameters, performing the initial Diffie-Hellman exchange, and implementing the cookie challenge mechanism for DoS protection. When the responder suspects a DoS attack (such as mass requests by the same source), it includes a cookie in the IKE_SA_INIT response. The initiator must return the cookie in its next request to prove that it truly exists at the IP address it claims, thereby mitigating resource exhaustion attacks.

This two-step exchange ensures the responder only allocates resources after successful proof of address, aligning with best security practices. Fortinet documentation confirms that this process occurs strictly in the IKE_SA_INIT phase, not in subsequent IKE_Auth or CHILD_SA exchanges.